In recent years, a so-called tandem type image forming apparatus including a plurality (four) of image forming stations has been proposed. Such an image forming apparatus is capable of quickly forming a color image using an electrophotographic process, and therefore, receives attention.
In each image forming station, around a photosensitive member, a charging device (charging means), an exposure device (exposure means) and developing device (developing means) are provided. Further, toner images formed at the respective image forming stations are successively transferred superposedly onto an intermediary transfer member (image receiving member), and thereafter, are transferred onto a recording material altogether.
Here, as a type of the charging device, two types, i.e., an “AC charging type” for (electrically) charging the photosensitive member by applying a superposed voltage of an AC voltage and a DC voltage and a “DC charging type” for (electrically) charging the photosensitive member by applying only the DC voltage have been known. The “AC charging type” is advantageous in that a surface of the photosensitive member can be uniformly charged compared with the “DC charging type”, but a discharge amount to the photosensitive member is large and therefore the photosensitive member tends to be liable to deteriorate. Further, an expensive AC voltage (power) source is needed. On the other hand, compared with the “AC charging type”, in the “DC charging type”, the photosensitive member does not readily deteriorate, but tends to be inferior in charging uniformity. That is, compared with the “DC charging type”, the “AC charging type” is high in initial costs and running costs. In other words, compared with the “AC charging type”, the “DC charging type” is advantageous in terms of the running costs and the initial costs.
Therefore, in the case where the “DC charging type” is intended to be employed, the following problems can generate.
Specifically, in an apparatus described in Japanese Laid-Open Patent Application 2002-189400, in order to lower a potential (residual potential) of the photosensitive member remaining after transfer to the neighborhood of 0 V, a device for optically discharging the photosensitive member, a so-called pre-exposure device (discharging means), is mounted. Thus, in the case where a method of discharging the photosensitive member by using the pre-exposure device is employed, in the charging device, the photosensitive member has to be charged from the neighborhood of 0 V to a desired potential (e.g., −700 V), and compared with the case where a discharging step by the pre-exposure device is not performed, a discharge current becomes large. That is, compared with the case where the discharging step by the pre-exposure device is not performed, the photosensitive member tends to be liable to deteriorate.